Hydroxyl radicals maintain the self-cleansing capacity of the troposphere

Abstract

Abstract. Billions of tons natural and anthropogenic gases are released and subsequently removed from the troposphere each year. Photochemical reactions, initiated by hydroxyl (OH) radicals, oxidise most gases to products which are more easily removed by precipitation and dry deposition at the Earth's surface. Since human-induced pollution emissions strongly affect OH formation and loss, large global changes in OH concentrations are possible. The available evidence as reviewed here, however, indicates that global mean OH has changed relatively little in the past century. Depletion of OH by reactive carbon gases has been compensated by increased OH formation by nitrogen oxides, an act of "inadvertent geo-engineering". Analyses of global mean OH for the past 2.5 decades, however, partly based on methyl chloroform measurements, are ambiguous. Especially the OH trend in the 1990s based on these analyses is at odds with present understanding. Since the discrepancy is dominated by the uncertainty in methyl chloroform emission estimates, improvements of source inventories and model analyses, combined with continued high precision methyl chloroform measurements, will help resolve this problem.

CTM study of changes in tropospheric hydroxyl distribution 1990–2001 and its impact on methane

“…A following reaction chain may be an efficient sink of OH although there is also a possibility of recycling through reaction of HO 2 with NO or O 3 , or photolysis of peroxides. Lelieveld et al [2002aLelieveld et al [ , 2004 calculates that this secondary formation is of similar magnitude as the primary and that nearly half of the OH initially lost in the oxidation of CH 4 and CO is recycled. The degree of recycling is very dependent on the NO x (NO + NO 2 ) levels.…”

Water-Soluble Oligomer Formation from Acid-Catalyzed Reactions of Levoglucosan in Proxies of Atmospheric Aqueous Aerosols

“…Recently, it was proposed that HULIS could be formed in cloudwater through aqueous-phase reactions of polar aromatic compounds with hydroxyl radicals (10). Because of its high reactivity, the hydroxyl radical is known to be the most important atmospheric oxidant as it governs the oxidation and removal of most trace gases (11) as well as various organic species in cloudwater (12). Within aqueous aerosols, the Fenton reaction may generate significant amounts of hydroxyl radicals through the reaction of hydrogen peroxide with Fe(II) under acidic conditions (12).…”

Gas-phase radical chemistry in the troposphere

“…The coupled nature of the chemical system may mean secondary production of OH (via reactions 14 and 17) with increasing NO x has offset increased primary loss owing to growing concentrations of reactive anthropogenic VOCs. 5 Other sources of HO x As previously mentioned, there are a number of other potential sources of HO x in the atmosphere, such as the photolysis of carbonyl compounds produced from the (partial) oxidation of hydrocarbon species. The simplest and most common of these is formaldehyde, which is produced from CH 4 oxidation (see Fig.…”

“…The bulk of the experimental observations below [NO x ] levels of 300 pptv fit with the P(O 3 ) dependency predicted by the model, but above [NO x ] y 300 pptv P(O 3 ), computed from the measured HO 2 and NO, continues to increase with NO x , suggesting a NO x -limited regime. An interesting feature of the dry upper troposphere in relation to radical formation is the role of acetone as a radical source 5,22,23 (See Table 1). From the preceding discussion it can be seen that the involvement of HO x radical chemistry in gas-phase tropospheric chemistry potentially has several significant consequences:…”

“…5). 5 Global ''OH'', a sometime used measure of oxidative capacity, has proven to be a difficult concept and has been developed to represent the diurnal mean ability of the atmosphere to oxidise trace compounds. The coupled nature of the chemical system may mean secondary production of OH (via reactions 14 and 17) with increasing NO x has offset increased primary loss owing to growing concentrations of reactive anthropogenic VOCs.…”